For the past several years, we have been investigating the role of latently infected, resting CD4+ T cells and persistent viral replication in the pathogenesis of HIV disease and the impact of this reservoir on the treatment of HIV-infected individuals. We previously demonstrated that the latent viral reservoir in the resting CD4+ T cell compartment persists in virtually all infected individuals receiving effective antiviral therapy. Consequently, this viral reservoir is a major impediment to the eradication of HIV in vivo. In addition, we realized that HIV continually replicates at low levels in chronically infected individuals who receive effective antiviral therapy that renders them consistently aviremic for prolonged periods of time. Over the past year, we have focused our research on: 1) delineation of the mechanism by which HIV persists in infected individuals receiving effective antiviral therapy for extended periods of time and 2) investigation of the dynamics of decay of viral reservoirs in infected individuals who initiate antiretroviral therapy at different stages of disease.[unreadable] First, we investigated the presence and status of residual HIV in individuals who had received effective antiretroviral therapy for prolonged periods of time and examined the underlying mechanisms by which HIV persists in CD4+ T cells of such individuals. We demonstrated that CD4+ T cells in the gut-associate lymphoid tissue (GALT) carried the highest level of HIV proviral DNA compared to CD4+ T cells in the blood of infected individuals receiving effective antiretroviral therapy for prolonged periods of time. Phylogenetic analyses of HIV env sequences revealed active cross-infection between blood- and GALT-associated CD4+ T cells. Furthermore, our data also suggest that intensification of antiretroviral therapy, especially through the addition of newer classes of anti-HIV drugs, will be necessary to contain low levels of on-going viral replication in the tissue compartment of chronically infected individuals in order to achieve complete clearance of virus in vivo.[unreadable] Second, we investigated the dynamics of decay of viral reservoirs in infected individuals who initiated antiretroviral therapy at different stages of disease. In a cross-sectional study, we demonstrated that the median copy number of HIV proviral DNA in subjects who initiated antiretroviral therapy within 6 months of infection was significantly lower compared to subjects initiating antiretroviral therapy during the chronic phase of infection (p=0.003). In order to examine the frequency of CD4+ T cells carrying infectious virus, a High Input Co-culture assay, which allows examination of large numbers of cells, was conducted using highly enriched CD4+ T cells from 8 infected individuals in whom no measurable HIV proviral DNA had been detected in their cells. The frequency of cells carrying infectious virus in HIV-infected individuals who initiated therapy within 6 months of infection was significantly lower compared to HIV-infected individuals who initiated therapy during the chronic phase of infection (p=0.03). Remarkably, no infectious virus could be recovered from the peripheral blood or GALT CD4+ T cells of one infected individual (<1 infected cell per 0.8x109 CD4+ T cells) in whom antiretroviral therapy was initiated early in infection. Our data suggest that the combination of initiation of antiretroviral therapy early in the course of HIV infection and prolonged suppression of viral replication can result in a profound diminution of HIV reservoirs that may lead to clearance of virus in infected individuals.